1. Field of the Invention
The present invention relates generally to air supply and exhaust control systems for internal combustion engines, and more particularly to a system for controlling air supply effected to an internal combustion engine by a plurality of turbosuperchargers accompanying with the internal combustion engine in substantially parallel with each other and exhaust discharged through an exhaust passage having variable exhaust resistance from the internal combustion engine.
2. Description of the Prior Art
In the field of internal combustion engines used in vehicles, there has been proposed a so-called sequentially controlled supercharging system in which a couple of turbosuperchargers of primary and secondary are employed in an internal combustion engine and so controlled that the engine is put in a first operating condition wherein only the primary turbosupercharger works for supercharging the engine or in a second operating condition where both the primary and secondary turbosuperchargers work simultaneously for supercharging the engine selected in accordance with exhaust gas flow from the engine, as disclosed in, for example, each of Japanese patent applications published before examination under publication numbers 56-41417, 59-160022, 60-259722, 61-275531 and 61-38124, Japanese utility model application published before examination under publication number 60-178329, and Japanese utility model application published after examination under publication number 57-12177. In such a system, the primary and secondary turbosuperchargers are provided with respective turbines disposed in portions of an exhaust passage of the engine which extend in substantially parallel with each other and respective blowers disposed in branch intake passages of the engine, and an exhaust cutoff valve is disposed in the portion of the exhaust passage through which exhaust gas is applied to the turbine of the secondary turbosupercharger to be closed so as to supply the turbine of the primary turbosupercharger with the exhaust gas collectively when the exhaust gas flow in the exhaust passage is relatively small and to be opened so as to supply both the turbines of the primary and secondary turbo-superchargers with the exhaust gas when the exhaust gas flow in the exhaust passage is relatively large. With such an operation of the exhaust cutoff valve, intake air compressed by the blower of the primary turbosupercharger or the blowers of the primary and secondary turbosuperchargers is supplied to a combustion chamber of the engine with required intake air mass flow responding to various operating conditions of the engine.
In the previously proposed sequentially controlled supercharging system constituted as mentioned above, the exhaust cutoff valve is controlled frequently to be open and closed selectively in response to variations in the exhaust gas flow so that the secondary turbosupercharger is caused to operate and to cease operating selectively in accordance with the opening and closing operation of the exhaust cutoff valve and therefore there is a fear of a control system for the exhaust cutoff valve being lowered in reliability. Accordingly, for avoiding such a fear, it may be considered to set a threshold value of the exhaust gas flow by which the exhaust cutoff valve is tuned into opening from closing to be relatively large so that an operating area provided on an operating characteristic chart of the engine, which may be shown with an axis of abscissa representing engine speed and an axis of ordinate representing engine load, wherein only the primary turbosupercharger is to work is extended and thereby to cause the exhaust cutoff valve to be reduced in frequency of opening and closing so that the control system for the exhaust cutoff valve is improved in reliability.
However, in the case where the threshold value of the exhaust gas flow by which the exhaust cutoff valve is tuned into opening from closing is set to be relatively large, exhaust pressure in the exhaust passage at a location downstream of the turbine of the primary turbosupercharger is increased excessively and thereby the primary turbosupercharger is undesirably compelled to work with reduced efficiency when the exhaust gas flow is relatively large.